The principle of flywheel energy storage is that a spinning wheel stores mechanical energy, energy that can be put in or taken out of the spinning wheel with the use of a motor or generator. The amount of energy stored in a flywheel depends on the mass of the rotor, the configuration of the rotor, and how fast the rotor is spinning. Specifically, the energy storage varies as a function of the rotational moment of inertia of the rotor and the square of the rotational speed of the rotor.
Flywheel energy storage depends on mechanical parts rotating in a precise relationship to electrical and other mechanical components. A major problem in these systems is the safety of the people and the property in the local area during the catastrophic failure of the rotating system. Historically, flywheels have been made of steel for the purpose of smoothing the flow of energy in the rotating machines. Steel flywheels, when stressed to failure by overspeed, will fracture into several large pieces. The inertial forces on the failed parts cause the parts to move radially outward away from the machine, at speeds proportional to the rotating speed of the flywheel before failure. This expulsion of energy can be extremely dangerous and destructive.
A new generation of flywheels is now being produced from composite materials (fiber and plastic) to take advantage of the composite material's inherent strengths which are much greater than steel. As a result, composite flywheels fail at much higher energy levels in quite a different manner than their steel counterparts. Instead of fracturing into pie-shaped pieces in the manner of steel flywheels, composite material flywheels fail such that a composite ring of circumferentially wrapped fibers extend as if the fibers were a viscous liquid. Although some fiber breakage occurs to initiate the expansion, the spinning mass of fibers remains grossly intact, while the plastic that binds the fibers together experiences complete failure.
Containment vessels for this type of composite material in failure have taken the form of very strong, rigid vessels. The practicality of making rigid vessels in large scale production is low and the space required for installation is prohibitive. These types of thick, rigid containment vessels have other disadvantages as well. First, containment vessels of this nature tend to be extremely heavy, and as such, are expensive and difficult to handle. Additionally, they cause the failed flywheel material fragments to divert their energy in the axial direction, since the rigid wall stops fragment expansion in the radial direction. Thus, this requires that containment vessels of this design utilize very heavy top and bottom end caps at the axial ends of the vessel, in order to contain diverted flywheel material fragments.
Another approach to safety in rotating materials, such as flywheels, is to overdesign and control the quality of the systems to the point that failure is exceedingly unlikely. This design philosophy is utilized in jet engine construction. Ideally, from a purely safety standpoint, this is the most desirable construction approach. However, if flywheels are to be widely utilized in diverse engineering applications, they cannot be as expensive to produce as jet engines.
Due to their superior strength qualities, flywheels constructed of composite materials may fail at speeds four to five times higher than that which was achievable using traditional steel flywheels. Thus, there is a continuing need for a relatively low cost, lightweight flywheel vessel that can contain flywheels that operate at energy levels on the order of twenty-five times higher than that produced by steel flywheels. Prior art physical mechanisms that have relied primarily on friction, local buckling, and pure tensile loading, have not proved to be sufficient since they cannot withstand the strain rate produced by the high speed event of the above-described type of failure. The material and configuration utilized in these types of prior art containment systems have not been able to change shape fast enough to avoid ultimate failure of the material.